Experimental vertical axis wind turbines (VAWT)

Test bed vertical axis wind turbine (VAWT)

I made this wind turbine as a test bed so I could easily trial different blade widths and sections.
The blades are cut from extruded polystyrene foam using a home made hot wire cutter with a battery charger as power.
I started with fairly large airfoil sections as they were easiest to cut and worked down in size.
The foam cores are covered in either thin fibreglass cloth and water based varnish or brown paper glued on with thinned PVA glue and sealed with varnish.
The plywood top and bottom arms of the rotor run on an extended central axle which support the blades at both ends.
The blades are fastened with chipboard screws into the foam core so they can be easily changed.

I went for a two blade design, rather than three, as it was quicker and easier to make the wider blades and they seem
more effective. A three blade design might start easier and run more smoothly.
The success of a particular set of blades reflect the how well the blades are matched to the characteristics of the alternator providing the load.

The overall dimensions of the wind turbine are:

Radius

27cm

Height

50cm

Number blades

2

Frontal area

0.27m2

The airfoil sections tried so far are listed below.

Blade chord

20cm

Section

Joukowski 22%

Plotted around circumference

Solidity

0.74

Blade area / frontal area

These were the first blades I made and I wanted something I knew would start and run. This wind turbine has no problems starting and will reach charging speed in quite light winds but it doesn't seem to be loaded by the generator. If the generator is disconnected when charging the rotor speed hardly increases and the central tell-tale barely blows out. It does run quite smoothly for a two blade turbine.

Blade chord

20cm

Section

Joukowski 22%

Plotted symmetrical

Solidity

0.74

Blade area / frontal area

Many Darrieus lift turbines are made with symmetrical blade airfoil sections where as the ones I had made were largely plotted along the path of rotation. These blades are the same airfoil section as the ones above but without the curved plot. Considering the chord is relatively large which resulted in a large difference in shape the was surprisingly little difference between the two sets of blades. These do not run as smoothly or free wheel so well between gusts when not charging, but the power output when charging seemed much the same.

Blade chord

15cm

Section

Joukowski 22%

Plotted around circumference

Solidity

0.56

Blade area / frontal area

These blades are the best so far of all the turbines I've made. The turbine will start quite easily and once started will turn faster than the wider blades both before and during charging. If the alternator is disconnected while charging the turbine will speed up markedly but they still seem to be able to accelerate and drive the alternator in gusts.

Blade chord

10cm

Section

Joukowski 22%

Plotted around circumference

Solidity

0.37

Blade area / frontal area

It looks like these blades are too narrow. They have only been tried in fairly low winds so far but they are reluctant to self start in winds that would not be a problem for the wider blades. I've given them a swing to get them going and this seems to work but a lull in the wind results in them just blowing round below wind speed.